This invention relates to apparatus and method of temperature compensating HFETs using a single power supply.
Advanced communication electronic devices, such as power amplifiers and the like, require high efficiency, high performance, and low current drain power semiconductor devices. Enhancement mode hetero-junction field effect transistors (HFETs) can provide single power supply operation, i.e., operate from a voltage of only one polarity. It is also desirable and a benefit in such device technology that a self-biasing capability can be provided for stabilizing device operation over temperature. A p/n junction or Schottky diode integrated with the HFET process can supply this need, The p/n junction diode, however, has a barrier potential more than 1 eV, which is too high for low voltage power amplifiers which have a drain supply voltage of about 3 volts. A turn-on voltage of about 0.6 volts or lower is desirable.
In the prior art, HFETs are fabricated with titanium tungsten nitride deposited as the gate metal contact on a stack of compound semiconductor material to form a high turn-on (e.g. 1.7 volts) Schottky barrier. Some attempts have been made to use a diode connected HFET (i.e. the gate forms one terminal and the common connected source and drain form the second terminal, as illustrated in FIG. 1) for temperature compensation in electronic devices. The major problems in using the diode connected HFET for temperature compensation are: the turn-on voltage is too high (approximately 1.7 V) for low voltage applications (some of which are required to operate at as low a supply voltage as 1.8 V) and the temperature coefficient is too low (approximately xe2x88x920.4 mV/degrees C.) which does not provide for adequate compensation over the entire operating temperature range (usually xe2x88x9250 degrees C. to 100 degrees C.) of the electronic devices. A larger negative temperature coefficient is required to compensate the electronic devices over the entire operating range.
Accordingly it is highly desirable to provide apparatus and a method of temperature compensating with a high temperature coefficient and a low turn-on voltage.